In U.S. Pat. No. 3,800,188 issued 26 Mar. 1974 I disclose a motor speed control that can be used in an industrial system, such as a bottling plant, in order to maintain the rotation speed of a motor output shaft within a predetermined range. This device basically comprises a dielectric synthetic-resin disk secured to the shaft and provided on its outer periphery with an undulating ferromagnetic ring. Spaced approximately 0.4 mm from this ring is a fixed detector head containing at least one permanent magnet and coil. As the shaft and disk rotate the undulations of the ring pass the detector head and induce a voltage in its coil. As the shaft and disk rotate, therefore, the coil produces a signal which is almost perfectly sinusoidal and which has a frequency which is directly proportional to the rotation of speed of the shaft.
This sinusoidal signal is passed through a resonant circuit which is tuned to a frequency corresponding to the desired rotational speed of the shaft. A differential amplifier and switching circuit are connected to this resonant circuit and to the motor. When the frequency fed to the resonant circuit rises above or below the frequency to which this circuit is tuned, its output drops off sharply and the switching means may operate so as to shut off the motor, increase its speed, or decrease its speed in order to compensate for the detected variation of the actual motor speed from the desired speed.